The insulin-receptor interaction: is the kinetic approach for inferring negative-cooperative site-site interactions valid?

An evaluation of the cross-linking model for the interaction of insulin with its receptor

The cross-linking model for insulin receptor interactions, in which a single insulin molecule may form a cross-link between an insulin receptor's alpha-subunits, has been expressed as a formal compartmental model and subjected to a systematic analysis, examining a number of predictions that have been made for this model. The kinetic parameters for the model were obtained by matching data from insulin receptor equilibrium binding studies and rates of formation of the insulin receptor complex. This analytical study has allowed a clear description of the kinetics of the ligand receptor complexes involved in such a mechanism. We conclude that the cross-linking model accounts for the anomaly of the 10-fold concentration difference in high- and low-affinity binding sites found when insulin binding is analyzed by conventional means. However, the phenomenon of acceleration of dissociation of labeled ligand by unlabeled ligand cannot be accounted for as an intrinsic part of the model. We suggest that this phenomenon arises from the destabilization of cross-link formation when a second insulin molecule binds.

Effects of growth hormone on fuel utilization and muscle glycogen synthase activity in normal humans

To examine the insulin antagonistic effects of growth hormone (GH), seven healthy subjects underwent, in random order, two 5-h euglycemic clamp studies with moderate hyperinsulinemia. A GH infusion (45 ng.kg-1.min-1) was given throughout one of the studies. GH inhibited the insulin-stimulated glucose disposal by 27% from 4.4 +/- 0.7 to 3.3 +/- 0.4 mg.kg-1.min-1 (P less than 0.02) and raised the nonprotein energy expenditures (NPEE) from 18.7 +/- 0.5 to 20.5 +/- 0.3 kcal.kg-1.24 h-1 (P less than 0.03). Lipid oxidation contributed 71.7 +/- 5.6% of NPEE during the GH infusion as compared with 48.7 +/- 5.2% during the control clamp (P less than 0.02). In skeletal muscle biopsies, insulin binding to wheat germ agglutinin-purified insulin receptors and insulin receptor kinase activity were unaffected by GH infusion. Glycogen synthase activation by insulin was inhibited by 41% during the GH clamp (fractional velocity 14.1 +/- 2.5 vs. 8.3 +/- 1.4%, P less than 0.03). In conclusion, GH 1) increases energy expenditures and inhibits glucose oxidation in favor of an increased lipid oxidation, and 2) inhibits insulin-mediated activation of the glycogen synthase in skeletal muscle biopsies by a mechanism distal to insulin receptor binding and kinase activity.

Demonstration of insulin induced enhancement of insulin dissociation from its receptor in H35 rat hepatoma cells in spite of a linear Scatchard plot

The dissociation of insulin from its receptor has been reported to be enhanced in the presence of unlabeled insulin. This phenomenon has been used as a proof for the existence of negative cooperativity. However, when insulin degradation was inhibited by the use of a combination of bacitracin and chloroquine or prevented by a short incubation time in insulin binding studies in H35 rat hepatoma cells, linear Scatchard plots were obtained. This indicates that in the absence of insulin degradation negative cooperativity does not occur in these cells. Still the dissociation of insulin from its receptor was enhanced in the presence of 167 nM unlabeled insulin. This seriously questions the use of demonstration of insulin induced enhancement of insulin dissociation from its receptor as proof for the existence of negative cooperativity.

Binding of epidermal growth factor (EGF) to a cultured human glioma cell line

We have studied binding of 125I-EGF to the human malignant glioma cell line U-343 MG aCl2:6, which is planned to be used as a model system in studies of toxic effects of EGF conjugates. Special care has been taken to fulfil the requirements for a correct Scatchard analysis of binding parameters. Binding as a function of time, temperature and pH was investigated as well as dissociation and internalization of bound EGF. The stability of EGF during incubation was also determined. After binding to the receptor, EGF is rapidly internalized and degraded at physiological temperature. We found that binding experiments should be performed at 4 degrees C, since at this temperature practically no internalization took place, whereas dissociation occurred. From displacement experiments using increasing concentrations of unlabelled EGF competing with 125I-EGF for binding, binding parameters were calculated using a computerized, nonlinear, least-squares regression analysis of binding data. We found that EGF bound to a class of high affinity receptors with an apparent dissociation constant KD of about 4 x 10(-10) M. The mean number of receptors was 25,000 per cell. In experiments where receptors were saturated with 125I-EGF an additional class of low affinity receptors was detected. This had an apparent KD of 1 x 10(-8) M with a mean receptor number per cell of 780,000. We also noticed enhanced dilution-induced dissociation of bound 125I-EGF in the presence of excess unlabelled EGF, suggesting negative cooperativity.

Exercise-enhanced activation of glycogen synthase in human skeletal muscle

The present study was undertaken to elucidate aspects of the regulatory mechanisms leading to enhanced glucose metabolism and insulin sensitivity of muscle after physical exertion. Biopsies were obtained from the vastus lateralis muscle of healthy volunteers before and after 60 min of bicycle exercise at 60% of their maximal aerobic capacity. Insulin binding to wheat germ agglutinin-purified muscle insulin receptors as well as basal and insulin-stimulated receptor kinase activity toward an exogenous substrate were unaltered by exercise. Muscle glycogen levels diminished from 3.35 +/- 0.26 to 1.85 +/- 0.13 mg/100 mg muscle (P less than 0.01) and the half-maximal activation constant of glycogen synthase for glucose 6-phosphate decreased from 0.62 +/- 0.05 to 0.25 +/- 0.02 mM (P less than 0.001). Total glycogen synthase activity was unchanged. In the absence of phosphatase inhibitors, glucose 6-phosphate-independent glycogen synthase activity of the crude enzyme extract increased during in vitro incubation. The initial rate of activation (through dephosphorylations) of glycogen synthase was 0.18 +/- 0.06 vs. 0.37 +/- 0.03 U.min-1.mg-1 protein before and after exercise, respectively (P less than 0.02). The total as well as the glycogen-associated phosphoprotein phosphatase activity was, however, unaffected by exercise.

Kinetics of insulin binding and kinase activity of the partially purified insulin receptor from human skeletal muscle

The kinetics of insulin binding and kinase activity of soluble, partially purified insulin receptors from human skeletal muscle are considered. An equilibrium for insulin binding was obtained within 2 h at 37 degrees C. At lower temperatures the equilibrium for insulin binding was less clearly defined. Dissociation of 125I-labelled insulin was incomplete unless an excess amount of unlabelled insulin was added. Insulin-stimulatable autophosphorylation of the 95 kDa subunit was verified by gel electrophoresis. The kinase activity was measured with the synthetic polypeptide poly(Glu-Tyr(4:1] as a phosphoacceptor. The insulin receptor kinase activity correlated significantly (r = 0.92, P less than 0.0001) to the concentration of high-affinity insulin binding sites in the eluate. Autophosphorylation of the insulin receptor was necessary for the activation of the receptor kinase. When activated the receptor kinase activity was stable for at least 60 min at 21 degrees C with a pH optimum of approx. 7.8, similar to the pH optimum for insulin binding. The non-ionic detergent Triton X-100 inhibited the sensitivity of the receptor kinase to insulin. Insulin stimulated the Vmax of the kinase reaction about 3-fold, decreased the Km for ATP from 35 +/- 5 microM (mean +/- S.E.) to 8 +/- 1 microM (P less than 0.02) and induced a positive cooperativity to ATP with an increase in the Hill coefficient from 1.00 +/- 0.02 to 1.37 +/- 0.07 (P less than 0.05). According to the Hill plots, insulin itself showed no cooperativity with respect to receptor binding or kinase activation.

Hepatic processing of insulin. Characterization of differential inhibition by weak bases

The effect of selected weak bases on the subcellular distribution and processing of internalized insulin by the liver has been studied. The effect of these bases on both the degradation products formed and on the kinetics of degradation have also been studied. 1. Methylamine, ammonium chloride and dansyl cadaverine but not chloroquine reduce the total amount of insulin endocytosed. 2. Ammonium chloride, dansyl cadaverine and chloroquine but not methylamine inhibit subsequent degradation and/or translocation of degradation products. 3. None of the weak bases changed the species of the degradation products found within the endocytic vesicles. 4. Kinetic analysis of intravesicular degradation indicates that dissociation from the receptor is the rate-limiting process in degradation. 5. Chloroquine and dansyl cadaverine but not methylamine or ammonium chloride showed specific inhibition of insulin degradation in isolated endocytic vesicles. 6. The effect of chloroquine and dansyl cadaverine on the kinetics of degradation suggest that they are acting by switching the receptor into a tight-binding conformation thereby slowing dissociation.

A short incubation time in insulin binding experiments leads to disappearance of negative cooperativity in H35 hepatoma cells

The effects of different periods of incubation (8 min vs 20 min) on insulin binding kinetics were examined in a H35 hepatoma cell line. Scatchard plots from cells incubated for 8 min were linear (r = 0.987 +/- 0.006), in contrast to curvilinear Scatchard plots from cells incubated for 20 min. Hill plots showed a slope of 1.006 +/- 0.024 for the 8 min incubation, whereas the slope was 0.827 +/- 0.0026 (p less than 0.0005) for the 20 min incubation. TCA precipitation of the medium showed minimal insulin degradation products at 8 min with a significant increase at 20 min (1.38 +/- 0.11% vs. 3.06 +/- 0.37%, p less than 0.0005). Internalized insulin was also significantly increased at 20 min as compared to 8 min incubation (48.9 +/- 5.6% vs. 32.4 +/- 3.0%, p less than 0.0005) These data indicate that after 8 min of incubation no appreciable cooperativity of insulin binding was present, while negative cooperativity was present after 20 min of incubation. As significantly more insulin degradation has taken place after prolonged incubation these data support the hypothesis that insulin degradation leads to negative cooperativity of insulin receptors.

Truncation of the ectodomain of the human insulin receptor results in secretion of a soluble insulin binding protein from transfected CHO cells

The insulin receptor is an integral transmembrane glycoprotein comprised of two alpha-(approximately 135 kDa) and two beta-(approximately 95 kDa) subunits, which is synthesized as a single polypeptide chain precursor (alpha beta). The primary sequence of the human insulin receptor (hIR) protein, deduced from the nucleotide sequence of cloned human placental mRNAs, predicts two large domains (929 and 403 residues) on either side of a single membrane spanning domain (23 residues); each of these major domains has a distinct function (insulin binding and protein/tyrosine kinase activity, respectively). To experimentally test this deduced topology, and to explore the potential for independent domain function by the hIR extracellular domain, we have constructed an expression plasmid encoding an hIR deletion mutant which is truncated 8 residues from the beginning of the predicted transmembrane domain (i.e., 921 residues). This domain of the hIR is in fact processed into alpha- and truncated beta-subunits and secreted with high efficiency from transfected CHO cell lines which express this mutant hIR, and the protein accumulates as an (alpha beta)2 dimer in the medium. This molecule is recognized by a battery of 13 monoclonal antibodies to epitopes on the IR extracellular domain, four of which block insulin binding and two of which require the native conformation of the IR for recognition. Further, this domain binds insulin with an apparent dissociation constant comparable to that of the wild-type hIR. However, the secreted dimer displays a linear Scatchard plot, while that of the wild-type membrane-associated hIR is curvilinear.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of specific insulin binding sites on chromaffin cells from bovine adrenal medulla

1. Insulin receptors were investigated in isolated chromaffin cells from bovine adrenal medulla. 2. The cells were incubated with [125I]insulin in HEPES buffer, pH 7.8 at 15 degrees C for 180 min to obtain steady state binding. Specific binding was linearly related to the number of cells in the range 0.5-10 x 10(6) cells/ml. Insulin and proinsulin caused half maximal displacement of specifically bound tracer in concentrations of 0.18 and 2.46 nM, respectively. 3. Computer analysis of the binding data gave a linear Scatchard plot, consistent with a single class of non-interacting receptors with an affinity constant of 5.6 nM-1, the total number of receptors per cell being 1700. 4. The apparent MW of the insulin binding subunit of the receptor was 135,000, determined by affinity crosslinking and SDS gel electrophoresis under reducing conditions.